In recent years, magnetic materials have been used for devices such as electromagnetic wave absorbers, magnetic inks, inductance elements and antenna devices, and their importance is increasing year by year. These components are used according to the properties, such as real part of magnetic permeability (real part of relative permeability) μ′ or imaginary part of magnetic permeability (imaginary part of relative permeability) μ″, of the magnetic material used. For example, inductance elements and antenna devices use a component having high μ′ (and also low μ″), while electromagnetic wave absorbers use a component having high μ″. Thus before a component is used actually as a device, μ′ and μ″ of the component should be adjusted to the frequency band of the device used. In recent years, the frequency band used by devices is becoming high, and there is thus an urgent need for a material having μ′ and μ″ values that can be adjusted at high frequency.
Magnetic materials having high μ′ and low μ″ values at high frequency are particularly highly expected in their application to devices for high-frequency communication products such as antenna devices. Current portable communication terminals use radio signals for most of information transmission. The frequency band of the radio signals currently used is a high-frequency region of 100 MHz or more. Thus, electronic parts and boards useful in the high-frequency region are attracting attention. Radio signals in a high-frequency range of the gigahertz band are used in mobile device communication and also in satellite communication.
As a method of reducing the size of the antenna and the power consumption, there is a method of performing communication by using an insulating substrate having high magnetic permeability (high μ′, low μ″) as the antenna board and thus preventing transmission of the radio signal from the antenna to electronic parts or boards in the communication device. More preferably, it is possible in this way to reduce the size of the antenna and the power and to expand the band of a resonant frequency of the antenna simultaneously.
When a high-magnetic-permeability material is used particularly for an antenna board, the thickness of the material should be 10 μm or more, preferably 100 μm or more. Currently, there is no insulative high-magnetic-permeability thick-film material having a thickness of 10 μm or more, preferably 100 μm or more that shows high magnetic permeability in a high-frequency band, particularly in the gigahertz band range. Thus, there is a demand, as an antenna board material, for a thick-film high-magnetic-permeability, insulative material (high μ′ and low μ″) that can be used for high-frequency radio signals and has a transmission loss as low as possible.
Electromagnetic wave absorbers absorb noise at a high frequency generated by electronic devices and reduce problems such as malfunction of the electronic devices, by using their high μ″. The electronic devices include semiconductor elements such as IC chips, various communication devices and the like. There are many kinds of electronic devices including those that are used in the frequency range of 1 MHz to several gigahertz, in particular in the high-frequency range of several tens of gigahertz or more.
For use of a magnetic material in high-frequency devices such as inductance elements, antennas and electromagnetic wave absorbers, efforts are made to flatten the shape of a magnetic material powder.
For example, JP-A H06-267723 (KOKAI) discloses a flat magnetic material powder containing soft magnetic metal particles having an aspect ratio, i.e., average major-axis length divided by average thickness, of 2 or more.
JP-A H06-267723 (KOKAI) describes that it is possible to raise the magnetic permeability at 100 kHz by flattening the particle. However, according to JP-A H06-267723 (KOKAI), because a flat particle has a large thickness of 100 μm, there is often eddy current generated, leading to increased magnetic loss by the eddy current loss, when an AC magnetic field is applied. Actually, the magnetic permeability of the particle at 100 kHz was studied in the Examples, and the magnetic loss at a frequency higher than that in the megahertz to gigahertz region becomes larger, and thus, it was considered that it cannot be used in high-frequency devices (such as inductance elements, antennas, electromagnetic wave absorbers, etc.).
Various materials have been proposed as the magnetic material for use at high frequency (high-magnetic-permeability material or wave absorber), but none of them satisfied the requirements in material properties (μ′, μ″, insulation properties, thickness, etc.).